Borowska-Wykret Dorota, Rypien Aleksandra, Dulski Mateusz, Grelowski Michal, Wrzalik Roman, Kwiatkowska Dorota
Department of Biophysics and Morphogenesis of Plants, Faculty of Biology and Environment Protection, University of Silesia in Katowice, Jagiellonska 28, 40-032 Katowice, Poland.
Institute of Material Science, University of Silesia in Katowice, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland.
Ann Bot. 2017 Jun 1;119(8):1365-1383. doi: 10.1093/aob/mcx015.
The capitulum of Helichrysum bracteatum is surrounded by scarious involucral bracts that perform hygroscopic movements leading to bract bending toward or away from the capitulum, depending on cell wall water status. The present investigation aimed at explaining the mechanism of these movements.
Surface strain and bract shape changes accompanying the movements were quantified using the replica method. Dissection experiments were used to assess the contribution of different tissues in bract deformation. Cell wall structure and composition were examined with the aid of light and electron microscopy as well as confocal Raman spectroscopy.
At the bract hinge (organ actuator) longitudinal strains at opposite surfaces differ profoundly. This results in changes of hinge curvature that drive passive displacement of distal bract portions. The distal portions in turn undergo nearly uniform strain on both surfaces and also minute shape changes. The hinge is built of sclerenchyma-like abaxial tissue, parenchyma and adaxial epidermis with thickened outer walls. Cell wall composition is rather uniform but tissue fraction occupied by cell walls, cell wall thickness, compactness and cellulose microfibril orientation change gradually from abaxial to adaxial hinge surface. Dissection experiments show that the presence of part of the hinge tissues is enough for movements.
Differential strain at the hinge is due to adaxial-abaxial gradient in structural traits of hinge tissues and cell walls. Thus, the bract hinge of H. bracteatum is a structure comprising gradually changing tissues, from highly resisting to highly active, rather than a bi-layered structure with distinct active and resistance parts, often ascribed for hygroscopically moving organs.
蜡菊的头状花序被干膜质的总苞片包围，这些总苞片会进行吸湿运动，导致苞片根据细胞壁水分状况朝向或远离头状花序弯曲。本研究旨在解释这些运动的机制。
使用复型法对运动过程中伴随的表面应变和苞片形状变化进行量化。解剖实验用于评估不同组织在苞片变形中的作用。借助光学显微镜、电子显微镜以及共聚焦拉曼光谱对细胞壁结构和组成进行研究。
在苞片铰链处（器官执行器），相对表面的纵向应变差异很大。这导致铰链曲率发生变化，从而驱动苞片远端部分的被动位移。远端部分在两个表面上又经历几乎均匀的应变以及微小的形状变化。铰链由类似厚壁组织的背面组织、薄壁组织和外壁增厚的正面表皮构成。细胞壁组成相当均匀，但细胞壁所占组织比例、细胞壁厚度、紧实度和纤维素微纤丝取向从铰链背面到正面表面逐渐变化。解剖实验表明，部分铰链组织的存在就足以实现运动。
铰链处的差异应变是由于铰链组织和细胞壁结构特征的正反梯度所致。因此，蜡菊的苞片铰链是一种由逐渐变化的组织构成的结构，从高度抗性到高度活性，而不是通常归因于吸湿运动器官的具有明显活性和抗性部分的双层结构。
